heightmap from textures and hight poly mesh to apply to low polly mesh?
The basics: 


Apply your texture as a normal map to your highpoly mesh.
Unwrap the lowpoly mesh and add an image to it (to bake to).
Select the highpoly, add the lowpoly to the selection (important to do this last). Bake normals.


With images:


Highpoly + lowpoly meshes





Test render of highpoly + texture





a voronoi texture applied as normal map





Bake setup and result.